1. Field of the Invention
The present invention relates to an ultrasonic motor which is used as a driving source of a machine such as a car.
2. Description of Related Art
A conventional ultrasonic motor has Langevin vibrators as shown by FIG. 8, and the Langevin vibrators are set such that their vibration rods 61 are slanted respect to a rotor 62. The vibration rod 61 of each Langevin vibrator 60 reciprocates in a direction of the axis of the vibrator 60, and because of the slanted arrangement, the tip of the vibration rod 61 moves in an elliptical motion. The elliptical motion of the vibration rods 61 drives the rotor 62 in one direction. In this method, however, there is a problem that the vibration rods 61 fatigue easily and have a short useful life.
In order to solve the problem, a traveling wave type ultrasonic motor 70 as shown by FIG. 9 has been suggested. The ultrasonic motor 70 has a ring stator 71, and on a lower surface of the ring stator 71, a plurality of piezoelectric ceramic plates 72 are arranged such that adjacent plates 72 will have mutually opposite polarization directions. Then, on an upper surface of the ring stator 71, a rotor 73 is provided. When the ultrasonic motor 70 is provided with two alternating voltages which have a phase difference of 90 degrees, the piezoelectric ceramic plates 72 vibrate in a shear mode, which excites a traveling flexural wave on the upper surface of the ring stator 71. Then, the rotor 73 is driven in one direction by the ring stator 71. The ring stator 71 and the rotor 73 are entirely in contact with each other, and therefore they do not get fatigued easily. In this method, however, since the shear vibration of the piezoelectric ceramic plates 72 is used, the efficiency is not good, and it is difficult to produce an ultrasonic motor with a large output.